Certain medical imaging modalities capture three-dimensional data. Examples of such tomographic imaging modalities include CT, MR, X-ray angiography with three-dimensional reconstruction, PET, SPECT, and other nuclear medicine imaging modalities, and newer forms of ultrasound acquisition. Image data obtained using such modalities can be presented conventionally as a series of two-dimensional images. However, increasingly it is seen as beneficial to store the three-dimensional data in a computer and generate two-dimensional images to extract different information, such as cross sections or projections, interactively under control of a user.
Various different types of images can be obtained from volumetric image data using known techniques, including Multi-Planar Reformat (MPR) images, Maximum Intensity Projection (MIP) images, and volume rendered (VR) images.
MPR images are two-dimensional cross-sectional images computationally extracted from a three-dimensional sampled field. The two-dimensional cross section can be an approximation to an infinitely thin plane. The cross section can also be a finite zone defined by a plane and a distance, where data is accumulated perpendicular to the plane using a maximum, minimum, or average operator. MPR images obtained using a cross section greater than a certain thickness are often referred to as MPR slabs. MPR slabs can be displayed using any suitable rendering technique (for example, intensity projections, volume rendering, intensity projection with colour map)
MIP images are projection images created by accumulating the entire three-dimensional field, or a large region of the three-dimensional field, along a vector. Usually, parallel vectors are used for accumulation and data is accumulated using a maximum, minimum, or average operator.
VR images are projection images obtained by accumulating the whole or a region of the three-dimensional field. Either parallel or divergent vectors are used for accumulation. The operator is chosen as a grossly approximate model of the interaction of coloured light and solid surfaces.
Many other types of two-dimensional or three-dimensional images can also be obtained, and MPR images, MIP images and VR images are mentioned purely by way of example.
In a known medical imaging system, it is known to present simultaneously a set of generated two-dimensional images, each two-dimensional image derived from the same three-dimensional data and/or the same measurement apparatus. For example, it is common to present three MPR images taken at mutually orthogonal planes, a VR image, and/or a MIP image. The user is able to modify geometric parameters of the images such as plane angles or locations, and photometric parameters such as the mapping from high-dynamic-range scalar data to display pixel values. New images are then generated according to the new parameters and the display is refreshed.
There is prior art relating to synchronized operation of groups of MPR images. For example, the display and synchronised manipulation of three mutually orthogonal MPR images is known, and various aspects of the arrangement, movement, or linkage of the MPR views with each other are implemented in known diagnostic imaging equipment.
One known technique is to present three mutually orthogonal MPR views such that the three-dimensional point where they intersect is aligned horizontally on the screen across two views, and vertically on the screen across two other views.
It is also known to present a VR or MIP view alongside the MPR views. However, the VR or MIP view in known systems is not well synchronized with the MPR views. In one common configuration known in the prior art, the VR or MIP view is controlled independently of the MPR views. A known method of controlling the field of view of the VR view is to present a three-dimensional wire frame cuboid superimposed on the VR or MIP view and allow the user to drag edges of the wire frame with a mouse to alter the VR or MIP field of view.
Another common configuration known in the prior art is to make the MPR views show a superset of the field of view of the VR or MIP view and allow the user to control a representation of the field of view of the VR or MIP view on the MPR views. A known way of doing this is to present on each MPR view a two-dimensional wire frame showing the orthogonal projection of the VR or MIP field of view in that MPR view, and to allow the user to drag the edges of the wire frame on the MPR to alter the field of view of the VR or MIP view.
A further technique, which is used by some known products, such as Toshiba's Voxar 3D® is to link the intersection point of the MPR views with the centre of the VR or MIP field of view. In this configuration the VR or MIP field of view may be constrained to be a cube. The user is able to move the intersection point of the MPR views by dragging cross hairs on the MPR views, by scrolling, or by other means. The VR or MIP field of view follows that movement as it is constrained to being always centred on the MPR intersection point.
Known techniques for displaying simultaneously three-dimensional images and two-dimensional images obtained from the same volumetric data set have various disadvantages.
For example, in the case of a system in which the user has to manipulate the VR or MIP view independently from the MPR views, the manual operations required of the user in order to view the same anatomical regions or features in MPR views and in a VR or MIP view can be tedious and time consuming. If the user wishes to continue to view the same anatomical regions or features as they navigate the images it is necessary for the user to continually manipulate the MPR views and the VR or MIP views and they have to manually ensure that fields of view of the different images overlap sufficiently.
In a system where the three-dimensional field of view of the VR or MIP views is controlled by a two-dimensional wire frame on the MPR views, it is difficult for a user to appreciate the anatomy of a subject in the MPR views or to make good use of screen space, as the MPR views have to have a wide field of view in order that the representation of the VR and MIP field of view can be freely positioned inside it. Furthermore, the method of controlling the field of view of the VR and MIP views in such a system is different from the method of controlling the field of view of MPR images (if such a method of controlling the field of view of the MPR is provided), which can make operation by a user complicated and time consuming.
Known systems in which the centre of the field of view of VR and MIP views is linked to the intersection point of the MPR allow the user to continually see the same anatomy in MPR and VR or MIP, at least over a small area, and it allows user control of the field of view on any of the MPR, VR, or MIP views. However such systems fail to make good use of screen space and sometimes fail to present the images according to standard conventions. For example it is common for such systems to display an MPR, VR or MIP field of view that is smaller than the available view and positioned off-centre, so that large portions of a view are unproductively blank.